This invention generally relates to electronic systems and in particular it relates to complementary bipolar operational amplifiers.
Resistors needed in the output stage to reduce resonances caused by emitter followers driving capacitive loads cause the current in the output stage to rise quiescently. The prior art output stage circuit of FIG. 1 shows such an output state. The prior art circuit of FIG. 1 includes transistors 10, 12, 14, and 16; resistors 18-21; current sources 22, 24, 26, and 28; capacitor 30; positive power supply node 32; negative power supply node 34; output node 36; and ground node 38. Current sources 22 and 24 each represents a signal dependent current from the input stage of a complementary bipolar operational amplifier. Current sources 22 and 24 provide the DC bias and the input signal to the circuit. Resistors 20 and 21 are needed to damp out the resonance formed by the emitter tuned inductance of transistors 10 and 12 and the base collector capacitances of transistors 14 and 16 and the capacitances of current sources 26 and 28. To counteract the effect of these resistors on the DC quiescent current, resistors 18 and 19 are added to subtract some voltage from between the bases of transistors 14 and 16. Resistors 18 and 19 need to be large enough to cause asymmetric phase delays from the gain node to the output. This generates additional stability problems that can lead to the output stage oscillating on its own.
Generally, and in one form of the invention, the output stage of a complementary bipolar operational amplifier includes: a first bipolar transistor; a second bipolar transistor coupled to the first bipolar transistor; a third bipolar transistor; a fourth bipolar transistor; a first resistor coupled between a base of the first bipolar transistor and the third bipolar transistor; a second resistor coupled between a base of the second bipolar transistor and the fourth bipolar transistor; a first current source; a second current source; a third resistor coupled between the first current source and the third transistor; a fourth resistor coupled between the second current source and the fourth transistor; a fifth resistor coupled between a base of the third transistor and a common node; a sixth resistor coupled between a base of the fourth transistor and the common node; a first input stage current source coupled to the base of the third transistor; and a second input stage current source coupled to the base of the fourth transistor.